1. Field of the Invention
The present invention generally relates to a system for providing termination power to a computer bus and, in particular, concerns a system which ascertains whether termination power is already being provided by other devices on the computer bus and, if not, provides termination power on an as needed basis.
2. Description of the Related Art
Presently, the standard bus for interconnecting computers and peripheral devices is the Small Computer System Interface (generally known as SCSI) bus which is a parallel multi-master input/output bus. A SCSI bus connected network can be viewed as a chain of devices linked together by the SCSI bus wherein the SCSI bus forms a general purpose interface between each of the devices. Generally, a SCSI bus network includes a host device and up to seven other peripheral or auxiliary devices wherein the host device can be comprised of a personal computer (PC) and the peripheral devices can include equipment such as printers, auxiliary information storage devices, CD ROM drivers, etc.
Signals that travel on the bus between devices have wave-like properties. For example, when the signals travelling along the bus come to an end or termination of the bus, these signals can be reflected back along the bus unless some system has been implemented to cancel the signals reaching the termination of the bus. Reflected signals travelling back along the bus can interfere with other signals carried on the bus possibly corrupting the data carried by the bus.
For this reason, on most SCSI busses, a termination is provided at the end of the bus which inhibits the reflection of signals on the bus. A typical prior art termination is provided by a pull-up pull-down resistor pack, known as a SCSI bus terminator, that performs the function of absorbing excess energies at the end of the bus and not allowing these energies to be reflected back, along the bus.
FIG. 1 illustrates a typical prior art SCSI bus terminator for a single data or control line of the SCSI bus. Similar bus terminators are generally provided for each of the data and control lines contained on the SCSI bus. This type of bus terminator requires 5 volts and 1 amp to operate. Another type of termination used on SCSI busses is an active terminator which provides signals to actively cancel signals on the data and control lines at the termination of the bus. One common type of active termination device is a UC 5601 small computer system interface active terminator available from Unitrode Integrated Circuits. These active terminators also require a significant amount of power, e.g., 2.9 volts. In the past, the power for either type of bus terminator has been provided by the host device, e.g., the personal computer. The standard desktop personal computer generally does not have any difficulty in providing termination power for the bus terminator as it usually receives power from a standard 120 VAC electrical outlet.
However, there are some situations where it is undesirable for the host device to be the source of termination power for a SCSI bus. Specifically, portable computers, notebook computers and palmtop computers are becoming increasingly popular and more powerful. Typically, these types of computers are battery powered. If one of these types of computers is used as a host device and is linked to peripheral devices via an information bus, such as a SCSI interface, and it is to provide the termination power, the resulting drain on the battery would significantly deplete the limited energy contained in the battery. In some circumstances, termination power requirements can use up to 45% of a battery's capacity. Hence, providing termination power from the battery can significantly reduce the battery's available power and thereby significantly reduce the use of the host device.
In some circumstances, however, one of the peripheral devices can be configured to provide the termination power. This can solve the problems associated with a battery operated host device having to provide termination power provided that the termination power provided by the host device is disabled or the host device is designed to never provide termination power. If more than one device is providing termination power to the SCSI bus terminator, the bus terminator can become unreliable and the devices providing the termination power can be damaged.
While some host devices can be configured to not provide termination power, this configuration generally requires making relatively complex adjustments to the host device, e.g., setting dip switches or modifying operating instructions. Hence, even if prior art systems can be configured so that the host device does not have to provide termination power, the system can't be readily changed in the event the host device has to provide termination power, such as when the peripheral device providing termination power fails or is removed from the bus. Further, if the host device is designed to never provide termination power, the flexibility of the host device to be used with some peripheral devices is significantly limited as many peripheral devices are not designed to provide termination power either.
Hence, there is a need for a system which can sense when a peripheral device is providing termination power to an information bus and disable the termination power provided by a host device when a peripheral device is providing the termination power. However, the system should be capable of providing termination power from the host device in the event that no peripheral device is capable of providing termination power. Further, the system should be flexible so that it can be used in conjunction with a network of devices with a minimal amount of reconfiguration of the devices being required.
In the event that the host device is required to provide termination power from a limited power source, it would be desirable to control the termination power from the host device so that the energy used to provide termination power is minimized. To this end, it would be desirable to have a system which would provide the termination power from a host device with a limited power source only when the termination power is actually needed to thereby conserve the energy in the limited power source.